Array of ultrasound transducers

ABSTRACT

The invention relates to an array of ultrasound transducers which each comprise a substrate ( 1 ), a membrane ( 2 ), a first electrode ( 4 ), a piezoelectric layer ( 5 ), and a second electrode ( 6 ), wherein the substrate ( 1 ) comprises at least one opening ( 3 ) which adjoins the membrane ( 2 ) at one side, while the piezoelectric layer ( 5 ) has a high piezoelectric coupling coefficient k and is textured. The invention further relates to an ultrasound transducer and to a method of manufacturing an array of ultrasound transducers.

[0001] The invention relates to an array of ultrasound transducers whicheach comprise a substrate, a membrane, a first electrode, apiezoelectric layer, and a second electrode, said substrate comprisingat least one opening which adjoins the membrane at one side. Theinvention further relates to an ultrasound transducer and to a method ofmanufacturing an ultrasound transducer.

[0002] The generation of ultrasonic sound takes place by purelymechanical means or by means of electroacoustic transducers whichutilize the magnetostrictive or piezoelectric effect. Since ultrasoundcan be easily realized technically nowadays, it is widely used. Thusultrasound is used for generating images in medical diagnostics or innon-destructive material testing.

[0003] The electroacoustical transducers used most widely are based onthe piezoelectric effect. In practice, one-dimensional ortwo-dimensional array systems are mostly used in addition tosingle-transducer systems. Two-dimensional array systems areparticularly interesting for the display of three-dimensional images.

[0004] The excitation of the piezoelectric elements in acoustictransducers takes place either in an AC field with a frequency of a fewkHz up to several MHz or, in particular in image generation, by shortoscillation bursts with a basic frequency of a few MHz and relativebandwidths of up to 100%. The excursion of the piezoelectric elements inthe field direction generates a continuous or pulsatory ultrasound wavein the coupled medium such as, for example, water or biological tissue.The reflections changing in dependence on the tissue density and thethroughput times changing with the path length are utilized for imagegeneration in medical diagnostics.

[0005] Piezoelectric ultrasound transducers are manufactured frompiezoelectric ceramic blocks nowadays. Piezoelectric ceramic blocks ofexactly defined dimensions and with electrodes on the upper and lowersides of the blocks are mounted in an array in a mounting preform.

[0006] It is a disadvantage of these conventional systems that thecontrol circuits must be separately constructed and cannot be integratedinto the system.

[0007] Less expensive ultrasound transducer array systems are formed bythe so-called piezoelectric micromachined ultrasound transducers (PMUT).The arrays of piezoelectric ultrasound transducers are provided directlyon silicon. Such an ultrasound transducer comprises besides a siliconsubstrate a membrane on which a first electrode, a piezoelectric layer,and a second electrode are provided. The membrane can be obtained simplythrough etching away of the silicon so that an opening is created. Thechange in length of the piezoelectric element excites the membrane intooscillation. To manufacture an array of such ultrasound transducers,several openings are generated for the creation of several membranes onone silicon substrate.

[0008] Such an ultrasound transducer is known from U.S. Pat. No.5,956,292. The material used for the piezoelectric layer is a ceramicmaterial such as, for example, ZnO, AlN, LiNbO₄,PbZr_(x)Ti_(1-x)O₃(0≦x≦1), BaTiO₃ or SrTiO₃.

[0009] For broadband frequency applications, however, materials with aparticularly high piezoelectric coupling coefficient k are required.

[0010] It is accordingly an object of the present invention to providean improved ultrasound transducer with a piezoelectric layer which has ahigh piezoelectric coupling coefficient k.

[0011] This object is achieved by means of an array of ultrasoundtransducers which each comprise a substrate, a membrane, a firstelectrode, a piezoelectric layer, and a second electrode, said substratecomprising at least one opening which adjoins the membrane at one side,wherein said piezoelectric layer is a textured layer.

[0012] A clear increase in the piezoelectric coupling coefficient k isachieved through texturing of the piezoelectric layer as compared with apolycrystalline piezoelectric layer.

[0013] It is preferred that the piezoelectric layer is a monocrystallinelayer.

[0014] An absolutely ideal texturized pattern is obtained if thepiezoelectric layer is formed by a monocrystalline layer.

[0015] It is furthermore preferred that the material of one electrode istextured.

[0016] The manufacture of a piezoelectric layer on a textured electroderenders it possible to provide the material of the piezoelectric layerin a textured manner.

[0017] It is particularly highly preferred that the piezoelectric layercomprises a material chosen from the group Pb(Zn_(⅓)Nb_(⅔))O₃—PbTiO₃,Pb(Mg_(⅓)Nb_(⅔))O₃—PbTiO₃, Pb(Ni_(⅓)Nb_(⅔))O₃—PbTiO₃,Pb(Sc_(⅓)Nb_(⅔))O₃—PbTiO₃,Pb(Zn_(⅓)Nb_(⅔))_(1-x-y)(Mn_(½)Nb_(½))_(x)Ti_(y)O₃ (0≦x≦1, 0≦y≦1),Pb(In_(½)Nb_(½))O₃—PbTiO₃, Sr₃TaGa₃Si₂O₁₄, K(Sr_(1-x)Ba_(x))₂Nb₅O₁₅(0≦x≦1), Na(Sr_(1-x)Ba_(x))₂Nb₅O₁₅ (0≦x≦1), BaTiO₃, (K_(1-x)Na_(x))NbO₃(0≦x≦1), (Bi,Na,K,Pb,Ba)TiO₃, (Bi,Na)TiO₃, Bi₇Ti₄NbO₂₁,(K_(1-x)Na_(x))NbO₃—(Bi,Na,K,Pb,Ba)TiO₃ (0≦x≦1),a(Bi_(x)Na_(1-x))TiO_(3−b)(KNbO_(3−c)){fraction (1/2)}(Bi₂O₃—Sc₂O₃)(0≦x≦1, a+b+c=1), (Ba_(a)Sr_(b)Ca_(c))Ti_(x)Zr_(1-x)O₃ (0≦x≦1, a+b+c=1),(Ba_(a)Sr_(b)La_(c))Bi₄Ti₄O₁₅ (a+b+c=1), Bi₄Ti₃O₁₂, LiNbO₃,La₃Ga_(5.5)Nb_(0.5)O₁₄, La₃Ga₅SiO₁₄, La₃Ga_(5.5)Ta_(0.5)O₁₄ andPbZr_(x)Ti_(1-x)O₃ (0≦x≦1) with and without dopants of La, Mn, Fe, Sb,Sr, Ni or combinations of these dopants.

[0018] It is advantageous that a barrier layer is provided on that sideof the substrate which faces the piezoelectric layer.

[0019] This barrier layer prevents chemical interactions between themembrane and the layer present thereon during processing of thetransducer.

[0020] It is preferred that the first and the second electrode aredisposed at opposite ends of the piezoelectric layer.

[0021] By placing the electrodes at opposite ends of the piezoelectriclayer a poled operation of the piezoelectric layer is enabled.

[0022] It is also preferred that the electrodes are disposed on the samesurface of the piezoelectric layer.

[0023] It is furthermore preferred that at least one further electrodeis disposed between the first and second electrodes.

[0024] The electrical impedance of the transducer is decreased by thiselectrodes arrangement.

[0025] It is advantageous that at least two electrodes are of the samepolarity and at least one electrode is of opposite polarity, whereinelectrodes of the same polarity are coupled in parallel.

[0026] By this means the electrical impedance of the transducer isreduced.

[0027] It is preferred that the electrodes form concentric rings.

[0028] The invention also relates to an array of ultrasound transducerswhich each comprise a substrate, a membrane, a first electrode, apiezoelectric layer, and a second electrode, said substrate comprisingat least one opening which adjoins the membrane at one side,characterized in that a barrier layer is provided on that side of themembrane which faces the piezoelectric layer.

[0029] This barrier layer prevents chemical interactions between themembrane and the layer present thereon during processing of thetransducer.

[0030] It is preferred that one of the electrodes is a texturedelectrode formed on the barrier layer.

[0031] It is also preferred that the barrier layer has a bendingstiffness which is less than that of the membrane layer.

[0032] It is advantageous that the distance between the neutral plane ofa transducer and the piezoelectric layer is larger than the thickness ofthe piezoelectric layer.

[0033] By this measures the effective coupling coefficient is enhanced.

[0034] It is also advantageous that the barrier layer is thicker thanthe piezoelectric layer.

[0035] The invention further relates to an ultrasound transducer whichcomprises a substrate, a membrane, a first electrode, a piezoelectriclayer, and a second electrode, the substrate comprising at least oneopening which adjoins the membrane at one side, wherein saidpiezoelectric layer is a textured layer.

[0036] The invention also relates to a method of manufacturing an arrayof ultrasound transducers which each comprise a substrate, a membrane, afirst electrode, a piezoelectric layer, and a second electrode, thesubstrate comprising at least one opening which adjoins the membrane atone side, wherein said piezoelectric layer is manufactured in a texturedmanner.

[0037] The invention will be explained in more detail below withreference to a drawing and ten embodiments. In the drawing

[0038]FIG. 1 shows the construction of an ultrasound transducer incross-section, and

[0039]FIG. 2 shows the construction of a further ultrasound transducerin cross-section.

[0040] In FIG. 1, a possible embodiment of an ultrasound transducercomprises a substrate 1 which may comprise, for example, silicon,silicon with (100) orientation or (111) orientation, MgO with (100)orientation, LaAlO₃, sapphire, GaAs, ceramic materials such as, forexample, ZrO₂ or Al₂O₃, ceramic materials such as, for example, ZrO₂ orAl₂O₃ each with a planarizing layer, glass-ceramic materials, or glassmaterials. A membrane 2, which may comprise an inorganic material, anorganic material, or a combination of inorganic and organic materials,is provided on the substrate 1. Inorganic materials used may be, forexample, SiC, SiO₂, polycrystalline silicon, Si₃N₄, or layered systemsmade of these materials. At least one opening 3 is created in thesubstrate 1 by means of etching or stamping. The opening 3 adjoins themembrane 2 at one side. The membrane 2 present on the opening 3 iscapable of oscillating thanks to this opening. If the substrate 1comprises a material with an orientation, for example MgO with a (100)orientation, it may be preferred that the opening 3 does not pass fullythrough the substrate 1 but that a thin layer remains and forms themembrane 2.

[0041] A first electrode 4 lies on the membrane 2, which electrodecomprises, for example, Pt with a layer thickness of 50 nm to 1 μm, Tiwith a layer thickness of 1 to 20 nm/Pt with a layer thickness of 20 to600 nm, Ti with a layer thickness of 1 to 20 nm/Pt with a layerthickness of 20 to 600 nm/Ti with a layer thickness of 1 to 20 nm, W,Ni, Ag, Mo, Au, Cu, Ti/Pt/Al, Ti/Ag, Ti/Ag/Ti, Ti/Ag/Ir, Ti/Ir, Ti/Pd,Ti/Ag_(1-x)Pt_(x) (0≦x≦1), Ti/Ag_(1-x)Pd_(x) (0≦x≦1), Ag_(1-x)Pt_(x)(0≦x≦1), Ti/Pt_(1-x) Al_(x) (0≦x≦1), Pt_(1-x)Al_(x) (0≦x≦1),Ti/Ag/Pt_(1-x)Al_(x) (0≦x≦1), Ti/Ag/Ru, Ru, Ru/RuO₂, Ti/Ru, Ti/Ir,Ti/Ir/IrO₂, Ti/Ag/Ir/IrO_(x) (0≦x≦2), Ti/Ag/Ru/RuO_(x) (0≦x≦2),Ti/Ag/Ru/Ru_(x),Pt_(1-x)(0≦x≦1), Ti/Ag/Ru/Ru_(x)Pt_(1-x)/RuO_(y) (0≦x≦1,0≦y≦2), Ti/Ag/Ru/RuO_(x)/Ru_(y)Pt_(1-y) (0≦x≦2, 0≦y≦1),Ti/Ag/Ru_(x)Pt_(1-x) (0≦x≦1), Ti/Ag/Pt_(x)Al_(1-x) (0≦x≦1),Pt_(x)Al_(1-x)/Ag/Pt_(y)Al_(1-y) (0≦x≦1, 0≦y≦1),Ti/Ag/Pt_(y)(RhO_(x))_(1-y) (0≦x≦2, 0≦y≦1), Ti/Ag/Rh/RhO_(x) (0≦x≦2),Ti/Ag/Pt_(x)Rh_(1-x) (0≦x≦1), Ti/Ag/Pt_(y)(RhO_(x))_(1-y)/Pt_(z)Rh_(1-z)(0≦x≦2, 0≦y≦1, 0≦z≦1), Ti/Ag_(x)Pt_(1-x)/Ir (0≦x≦1),Ti/Ag_(x)Pt_(1-x)/Ir/IrO_(y) (0≦x≦1, 0≦y≦2),Ti/Ag_(x)Pt_(1-x)/Pt_(y)Al_(1-y) (0≦x≦1, 0≦y≦1), Ti/Ag_(x)Pt_(1-x)/Ru(0≦x≦1), Ti/Ag_(x)Pt_(1-x)/Ru/RuO_(y) (0≦x≦1, 0≦y≦2), Ti/Ag/Cr,Ti/Ag/Ti/ITO, Ti/Ag/Cr/ITO, Ti/Ag/ITO, Ti/Ni/ITO, Ti/Rh, Ti/Rh/RhO₂,Ti/Ni/Al/ITO, Ti/Ni, Ti/W/Ti, W_(x)Ti_(1-x) (0≦x≦1),W_(x)Ti_(1-x)/Al(Cu) (0≦x≦1), W_(x)Ti_(1-x)/Al(Si) (0≦x≦1),W_(x)T_(1-x)/Al(0≦x≦1) or Ti/Cu. A piezoelectric layer 5 is provided onthe first electrode 4. The layer thickness of the piezoelectric layer 5preferably lies between 1 and 50 μm. To achieve higher bandwidths,materials with a high piezoelectric coupling coefficient k are used inthe piezoelectric layer 5. A second electrode 6 is provided on thepiezoelectric layer 5, which electrode comprises, for example, Al, Aldoped with Si, Al doped with Cu, Pt with a layer thickness of 50 nm to 1μm, Ti with a layer thickness of 1 to 20 nm/Pt with a layer thickness of20 to 600 nm, Ti with a layer thickness of 1 to 20 nm/Pt with a layerthickness of 20 to 600 nm/Ti with a layer thickness of 1 to 20 nm, W,Ni, Ag, Mo, Au, Cu, Ti/Pt/Al, Ti/Ag, Ti/Ag/Ti, Ti/Ag/Ir, Ti/Ir, Ti/Pd,Ti/Ag_(1-x)Pt_(x) (0≦x≦1), Ti/Ag_(1-x)Pd_(x) (0≦x≦1), Ag_(1-x)Pt_(x)(0≦x≦1), Ti/Pt_(1-x)Al_(x) (0≦x≦1), Pt_(1-x)Al_(x) (0≦x≦1),Ti/Ag/Pt_(1-x)Al_(x) (0≦x≦1), Ti/Ag/Ru, Ru, Ru/RuO₂, Ti/Ru, Ti/Ir,Ti/Ir/IrO₂, Ti/Ag/Ir/IrO_(x) (0≦x≦2), Ti/Ag/Ru/RuO_(x) (0≦x≦2),Ti/Ag/Ru/Ru_(x)Pt_(1-x) (0≦x≦1), Ti/Ag/Ru/Ru_(x)Pt_(1-x)/RuO_(y) (0≦x≦1,0≦y≦2), Ti/Ag/Ru/RuO_(x)/Ru_(y)Pt_(1-y) (0≦x≦2, 0≦y≦1),Ti/Ag/Ru_(x)Pt_(1-x) (0≦x≦1), Ti/Ag/Pt_(x)Al_(1-x) (0≦x≦1),Pt_(x)Al_(1-x)/Ag/Pt_(y)Al_(1-y) (0≦x≦1, 0≦y≦1),Ti/Ag/Pt_(y)(RhO_(x))_(1-y) (0≦x≦2, 0≦y≦1), Ti/Ag/Rh/RhO_(x) (0≦x≦2),Ti/Ag/Pt_(x)Rh_(1-x) (0≦x≦1), Ti/Ag/Pt_(y)(RhO_(x))_(1-y)/Pt_(z)Rh_(1-z)(0≦x≦2, 0≦y≦1, 0≦z≦1), Ti/Ag_(x)Pt_(1-x)/Ir (0≦x≦1),Ti/Ag_(x)Pt_(1-x)/Ir/IrO_(y) (0≦x≦1, 0≦y≦2),Ti/Ag_(x)Pt_(1-x)/Pt_(y)Al_(1-y) (0≦x≦1, 0≦y≦1), Ti/Ag_(x)Pt_(1-x)/Ru(0≦x≦1), Ti/Ag_(x)Pt_(1-x)/Ru/RuO_(y) (0≦x≦1, 0≦y≦2), Ti/Ag/Cr,Ti/Ag/Ti/ITO, Ti/Ag/Cr/ITO, Ti/Ag/ITO, Ti/Ni/ITO, Ti/Rh, Ti/Rh/RhO₂,Ti/Ni/Al/ITO, Ti/Ni, Ti/W/Ti, W_(x)T_(1-x) (0≦x≦1), W_(x)Ti_(1-x)/Al(Cu)(0≦x≦1), W_(x)Ti_(1-x)/Al(Si) (0≦x≦1), W_(x)Ti_(1-x)/Al (0≦x≦1) orTi/Cu. In alternate embodiments other than that shown in the drawings,the electrodes 4,6 may be disposed at laterally opposite ends of thepiezoelectric layer 5, for laterally poled operation of thepiezoelectric layer 5. Additional electrodes may be be laterallydispersed between the ends of the piezoelectric layer 5 to decrease theelectrical impedance of the transducer. For instance, four electrodesmay be formed at discrete locations across a lateral surface of thepiezoelectric layer 5, with alternate electrodes being of alternatepolarity and electrodes of the same polarity being coupled in parallelfor reduced electrical impedance.

[0042] It may be advantageous that a barrier layer 7 is provided betweenthe membrane 2 and the first electrode 4. This barrier layer 7 preventspotential chemical interactions between the membrane 2 and the layerspresent thereon, for example a first electrode 4 or a piezoelectriclayer 5. The barrier layer 7 may comprise, for example, Al₂O₃, ZrO₂,TiO₂, HfO₂, MgO or LaAlO₃, including a composite of several of thesematerials. In addition, a textured electrode may be readily formed onthe barrier layer 7. The barrier layer 7 preferably has a bendingstiffness which is substantially less than that of the membrane layer.This property, together with the thickness of the barrier layer 7,enables the distance between the neutral plane of the transducer and thepiezoelectric layer 5 to be somewhat larger than the thickness of thepiezoelectric layer 5, which improves the effective coupling coefficientof the transducer. Perferably the barrier layer 7 is thicker than thepiezoelectric layer 5.

[0043] A voltage can be applied to the piezoelectric layer 5 through afirst current supply contact 8 and a second current supply contact 9,each connected to a respective electrode 4, 6, and the piezoelectriclayer 5 may thus be brought into oscillation. If the piezoelectric layer5 is excited by acoustic energy, the resulting voltage can be taken offthrough the first and second current supply contacts 8, 9 and measured.

[0044] A protective layer of an organic or inorganic material, or acombination of these materials, may be provided over the entireultrasound transducer. The organic material used may be, for example,polybenzocyclobutene or polyimide, and the inorganic material may be,for example, Si₃N₄, SiO₂ or Si_(x)O_(y)N_(z) (0≦x≦1, 0≦y≦1, 0≦z≦1).

[0045] The material in the piezoelectric layer 5 is textured, preferablywith a column-type growth, for achieving high piezoelectric couplingcoefficients k. The crystals of the piezoelectric material in a texturedpiezoelectric layer 5 have a preferred direction with respect to anexternal sampling geometry and are not statistically randomly arrangedwith respect to one another as regards their crystallographicorientation.

[0046] A textured piezoelectric layer 5 may be achieved by means ofthin-film processes such as, for example, sol-gel processes, sputtering,CVD (Chemical Vapor Deposition) processes, or printing techniques. Thepiezoelectric layer 5 may also be provided on the first electrode 4through bonding or gluing of previously sintered and mechanicallyprocessed layers of the piezoelectric materials.

[0047] A further alternative is formed by the so-called templated graingrowth (DGG) method. In this method, a polycrystalline layer is providedon a monocrystalline template. The polycrystalline layer is convertedinto a textured monocrystalline layer through heating at hightemperatures.

[0048] Materials which may be used for the piezoelectric layer 5 are,for example, ferroelectric materials, electrostrictive materials, aswell as special piezoelectric materials. Thus, for example,Pb(Zn_(⅓)Nb_(⅔))O₃—PbTiO₃, Pb(Mg_(⅓)Nb_(⅔))O₃—PbTiO₃,Pb(Ni_(⅓)Nb_(⅔))O₃—PbTiO₃, Pb(Sc_(⅓)Nb_(⅔))O₃—PbTiO₃,Pb(Zn_(⅓)Nb_(⅔))_(1-x-y)(Mn_(½)Nb_(½))_(x)Ti_(y)O₃ (0≦x≦1, 0≦y≦1),Pb(In_(½)Nb_(½))O₃—PbTiO₃, Sr₃TaGa₃Si₂O₁₄, K(Sr_(1-x)Ba_(x))₂Nb₅O₁₅(0≦x≦1),Na(Sr_(1-x)Ba_(x))₂Nb₅O₁₅ (0≦x≦1), BaTiO₃, (Ka_(1-x)Na_(x))NbO₃(0≦x≦1), (Bi,Na,K,Pb,Ba)TiO₃, (Bi,Na)TiO₃, Bi₇Ti₄NbO₂₁,(K_(1-x)Na_(x))NbO₃—(Bi,Na,K,Pb,Ba)TiO₃ (0≦x≦1),a(Bi_(x)Na_(1-x))TiO₃—b(KNbO_(3−c))—{fraction (1/2)}(Bi₂O₃—Sc₂O₃)(0≦x≦1, a+b+c=1), (Ba_(a)Sr_(b)Ca_(c))Ti_(x)Zr_(1-x)O₃ (0≦x≦1, a+b+c=1),(Ba_(a)Sr_(b)La_(c))Bi₄Ti₄O₁₅ (a+b+c=1), Bi₄Ti₃O₁₂, LiNbO₃,La₃Ga_(5.5)Nb_(0.5)O₁₄, La₃Ga₅SiO₁₄, La₃Ga_(5.5)Ta_(0.5)O₁₄ andPbZr_(x)Ti_(1-x)O₃ (0≦x≦1) with and without dopants of La, Mn, Fe, Sb,Sr, Ni or combinations of these dopants may be used in the piezoelectriclayer 5.

[0049] The structured deposition of the piezoelectric layer 5 on anoriented first electrode 4, on a textured barrier layer 7, or a texturedmembrane 2 renders it possible to influence the texture of thepiezoelectric layer 5.

[0050] Thus it is possible, for example, to grow a piezoelectric layerwith a (111) orientation through the use of a first electrode 4 of Ptwith a (111) orientation which is not thermally pre-treated. The use ofa thermally pre-treated first electrode of Pt with a (111) orientationrenders it possible to grow piezoelectric layers with a (100) and (111)orientation, or exclusively with a (100) orientation.

[0051] Alternative methods may be used for influencing the growth of thematerials of the piezoelectric layer 5 besides the deposition on athermally pre-treated first electrode 4 of Pt with (111) orientation.

[0052] The use of a membrane 2 or a barrier layer 7 of MgO with a (100)orientation enables the growth first of (100) oriented Pt as the firstelectrode 4. As a result, the piezoelectric materials listed above canbe grown on the textured first electrode 4 in the form of a texturedlayer.

[0053] The texture of the piezoelectric layer 5 can be improved furthertowards the monocrystalline state through further processes such as, forexample, laser annealing.

[0054] To improve the growth, the crystallinity, and the orientation ofthe material of the piezoelectric layer 5, a nucleating layer may beprovided on the first electrode 4.

[0055] A further possibility for enhancing the piezoelectric couplingfactor k is the polarization of the ceramic material in thepiezoelectric layer 5. The ceramic material can be polarized in thefield direction through the application of a bias voltage to the firstand second electrodes 4, 6.

[0056] In the embodiment of an ultrasound transducer shown in FIG. 1,the first electrode 4 is arranged below the piezoelectric layer 5 andthe second electrode 6 above the piezoelectric layer 5. The applicationof an AC voltage to the electrodes 4, 6 through the first and secondcurrent supply contacts 8, 9 causes the piezoelectric layer 5 to beexcited into a longitudinal or transverse oscillation perpendicular tothe layer thickness.

[0057] Alternatively, the electrodes 4, 6 may be provided in structuredmanner on only one side of the piezoelectric layer 5, for example on theside opposed to the membrane 2. In this case, the application of an ACvoltage will excite the piezoelectric layer 5 into a longitudinaloscillation in the plane of the layer. A nucleating layer may beprovided on the barrier layer 7 in this case for improving the growth,the crystallinity, and the orientation.

[0058] Alternatively, the substrate 1 may comprise on its rear side aninsulating layer of SiO₂ or Si₃N₄ or a combination of these materials.

[0059] A plurality of such ultrasound transducers may be provided on asubstrate 1. A one-dimensional or two-dimensional array of ultrasoundtransducers can be manufactured through a suitable electrical connectionof the individual ultrasound transducers. The piezoelectric layer 5, thefirst and second electrodes 4, 6, as well as the nucleating layer, ifpresent, are structured in such a manner in this case that theindividual ultrasound transducers are spatially separated from oneanother.

[0060] A membrane 2 is prepared on a substrate 1 for manufacturing anarray of ultrasound transducers. For this purpose, a layer of a suitablematerial may be deposited on the substrate 1. Openings 3 are created inthe substrate 1 by means of an etching process, which openings passfully through the substrate 1 and are bounded at one side by themembrane.

[0061] Alternatively, for example, the substrate 1 may be etched awayonly partly, so that a thin layer remains and constitutes the membrane2.

[0062] A barrier layer 7 may be provided on the membrane 2. A firstelectrode 4 is then provided on the membrane 2 or on the barrier layer7, as applicable, wherever an opening 3 is present in the substrate. Atextured piezoelectric layer 5 is provided on each first electrode 4,and a second electrode 6 is provided on each piezoelectric layer 5. Thefirst and second electrodes 4, 6 are then provided with a first andsecond current supply contact 8, 9, respectively.

[0063] Alternatively, an opening 3 may extend partly into the membrane2. For this purpose, portions of the membrane 2 lying in mutuallyopposed regions of an opening 3 are removed, for example by means ofetching or sputtering. The resonance frequency or the acoustic impedanceof the membrane 2 can be adapted to the medium to be acousticallyvibrated by this measure.

[0064]FIG. 2 shows a further embodiment of an ultrasound transducer. Inthis embodiment, portions of the membrane 2 lying in mutually opposedregions of an opening 3 have been removed, for example by means ofetching or sputtering. Furthermore, the layers lying on the membrane 2such as, for example, the barrier layer 7, the first electrode 4, thepiezoelectric layer 5, and the second electrode 6, are at most as wideas the region of the membrane 2 which adjoins an opening 3. The layersare thus connected to the substrate 1 in a cross-section takenperpendicularly to FIG. 2.

[0065] The manufacture of an array of ultrasound transducers comprisinga substrate 1 of a ceramic material may also take place in analternative manner. To this end, first the substrate 1 is manufacturedin a film casting process from a ceramic powder such as, for example,ZrO₂ or Al₂O₃, and an organic binder. Openings are made in the substrate1 in a stamping process in a number as required for the array ofultrasound transducers. Then the substrate is fully covered with a filmof a ceramic material, laminated, and sintered. A first electrode 4 isprovided on the sintered membrane 2 wherever there is an opening 3 inthe substrate 1. A nucleating layer, for example comprising Bi₄Ti₃O₁₂,is provided on each electrode 4. The nucleating layer covers the firstelectrode 4 only partly, preferably over 10% thereof. Then the ceramicmaterial of the piezoelectric layer 5 is provided on each nucleatinglayer, preferably in a thick-film process, and is sintered in a texturedmanner at 1275° C. Which orientation will be present in thepiezoelectric layer 5 depends on the exact composition and doping of theceramic material. Finally, the second electrode 6 is provided on eachpiezoelectric layer 5, and each electrode 4, 6 is provided with arespective current supply contact 8, 9.

[0066] Alternatively, other methods may be used for manufacturing atextured piezoelectric layer 5, for example the use of a texturedsubstrate 1, a textured barrier layer 7, or a textured first electrode4.

[0067] Alternatives in the construction of an array of ultrasoundtransducers or one ultrasound transducer and in the shaping of thevarious layers and/or the opening 3 are familiar to those skilled in theart.

[0068] Embodiments of the invention will be explained in more detailbelow, representing examples of how the invention may be realized inpractice.

[0069] Embodiment 1

[0070] An array of ultrasound transducers comprises a silicon substrate1 which has an insulating layer of Si₃N₄ and SiO₂ on one side. On theopposite side, a membrane 2 composed of a layered system ofSiO₂/Si₃N₄/SiO₂ is provided. The substrate 1 has a plurality of openings3 which each adjoin the membrane 2 at one side. A barrier layer 7 ofTiO₂ is present on the membrane 2. A first electrode 4 of Ti/Pt isprovided on the barrier layer 7 wherever an opening 3 was etched intothe substrate 1. The Pt-containing layer of the first electrode 4 isgrown so as to have a (111) orientation. A layer ofPbZr_(0.35)Ti_(0.65)O₃ with (100) orientation and (111) orientation isprovided on each first electrode 4 so as to form the piezoelectric layer5. A second electrode 6 of Pt lies on each piezoelectric layer 5. Thefirst and second electrodes 4, 6 of each ultrasound transducer areconnected to a first and second current supply contact 8, 9,respectively. The individual ultrasound transducers are electricallyconnected on the substrate 1 such that a one-dimensional array ofultrasound transducers is obtained. The application of a bias voltage toeach individual ultrasound transducer at elevated temperatures polarizesthe ceramic material in the piezoelectric layer 5 in the fielddirection. An array of ultrasound transducers is obtained thereby inwhich each ultrasound transducer has a very high piezoelectric couplingcoefficient k.

[0071] Embodiment 2

[0072] An array of ultrasound transducers comprises a silicon substrate1 which has an insulating layer of Si₃N₄ and SiO₂ on one side. On theopposite side, a membrane 2 composed of a layered system ofSiO₂/polycrystalline silicon/SiO₂ is provided. The substrate 1 has aplurality of openings 3 which each adjoin the membrane 2 at one side. Abarrier layer 7 of TiO₂ is present on the membrane 2. A first electrode4 of Ti/Pt is provided on the barrier layer 7 wherever an opening 3 wasetched into the substrate 1. The Pt-containing layer of the firstelectrode 4 is grown so as to have a (111) orientation. A thinnucleating layer of PbZr_(0.35)Ti_(0.65)O₃ is provided on each firstelectrode 4. A layer of Pb(Zn_(⅓)Nb_(⅔)) O₃—PbTiO₃ with a (001)orientation is grown on each nucleating layer so as to form thepiezoelectric layer 5. A second electrode 6 of Pt lies on eachpiezoelectric layer 5. The first and second electrodes 4, 6 of eachultrasound transducer are connected to a first and second current supplycontact 8, 9, respectively. The individual ultrasound transducers areelectrically connected on the substrate 1 such that a one-dimensionalarray of ultrasound transducers is obtained. The application of a biasvoltage to each individual ultrasound transducer at elevatedtemperatures polarizes the ceramic material in the piezoelectric layer 5in the field direction. An array of ultrasound transducers is obtainedthereby in which each ultrasound transducer has a very highpiezoelectric coupling coefficient k.

[0073] Embodiment 3

[0074] An array of ultrasound transducers comprises a silicon substrate1 which has an insulating layer of Si₃N₄ and SiO₂ on one side. On theopposite side, a membrane 2 composed of a layered system ofSiO₂/polycrystalline silicon/SiO₂ is provided. The substrate 1 has aplurality of openings 3 which each adjoin the membrane 2 at one side. Abarrier layer 7 of TiO₂ is present on the membrane 2. A first electrode4 of Ti/Pt is provided on the barrier layer 7 wherever an opening 3 wasetched into the substrate 1. The Pt-containing layer of the firstelectrode 4 is grown so as to have a (111) orientation. A thinnucleating layer of PbZr_(0.35)Ti_(0.65)O₃ is provided on each firstelectrode 4. A layer of Pb(Mg_(⅓)Nb_(⅔)) O₃—PbTiO₃ with a (001)orientation and with a (111) orientation is grown on each nucleatinglayer so as to form the piezoelectric layer 5. A second electrode 6 ofPt lies on each piezoelectric layer 5. The first and second electrodes4, 6 of each ultrasound transducer are connected to a first and secondcurrent supply contact 8, 9, respectively. The individual ultrasoundtransducers are electrically connected on the substrate 1 such that aone-dimensional array of ultrasound transducers is obtained. Theapplication of a bias voltage to each individual ultrasound transducerat elevated temperatures polarizes the ceramic material in thepiezoelectric layer 5 in the field direction. An array of ultrasoundtransducers is obtained thereby in which each ultrasound transducer hasa very high piezoelectric coupling coefficient k.

[0075] Embodiment 4

[0076] An array of ultrasound transducers has a substrate 1 of siliconwhich supports an insulating layer of Si₃N₄ and SiO₂ on one side. On theopposite side, a membrane 2 formed by a layered system ofSiO₂/polycrystalline silicon/SiO₂ is provided. The substrate 1 has aplurality of openings 3 which adjoin the membrane at one side. Portionsof the membrane 2 lying in mutually opposed regions of an opening 3 havebeen removed by means of etching. A barrier layer 7 of ZrO₂ is presenton the membrane 2. A first electrode 4 of Ti/Pt is provided on thebarrier layer 7 wherever there is an opening 3 in the substrate 1. ThePt-containing layer of the first electrode 4 is grown so as to have a(111) orientation. A thin nucleating layer of PbZr_(0.35)Ti_(0.65)O₃ isprovided on each first electrode 4. A layer of Pb(Ni_(⅓)Nb_(⅔))O₃—PbTiO₃ with a (001) orientation and with a (111) orientation is grownon each nucleating layer so as to form the piezoelectric layer 5. Asecond electrode 6 of TiW/Al lies on each piezoelectric layer 5. Thelayers present on the membrane are at most as wide as the membrane 2 inthe region of an opening 3. The first and second electrodes 4, 6 of eachultrasound transducer are provided with a first and a second currentsupply contact 8, 9, respectively. The individual ultrasound transducersare electrically connected on the substrate 1 in such a way that aone-dimensional array of ultrasound transducers is obtained. Thisresults in an array of ultrasound transducers in which each ultrasoundtransducer has a very high piezoelectric coupling coefficient k.

[0077] Embodiment 5

[0078] An array of ultrasound transducers comprises a silicon substrate1 which comprises an insulating layer of Si₃N₄ and SiO₂ on one side. Onthe opposite side, a membrane 2 composed of a layered system ofSiO₂/polycrystalline silicon/SiO₂ is provided. The substrate 1 has aplurality of openings 3 which adjoin the membrane 2 at one side. Abarrier layer 7 of MgO with a (100) orientation is present on themembrane 2. A thin nucleating layer of PbZr_(0.35)Ti_(0.65)O₃ isprovided on the barrier layer 7 wherever there is an opening 3 in thesubstrate 1. A layer of Pb(Zn_(⅓)Nb_(⅔))O₃—PbTiO₃ with a (001)orientation is provided as the piezoelectric layer 5 on each nucleatinglayer. A first and a second electrode 4, 6 of TiW/Al are present on eachpiezoelectric layer 5. The two electrodes 4, 6 are structured in such away that two electrode surfaces on the upper side of the piezoelectriclayer 5 form an ultrasound transducer each time. The first and secondelectrodes 4, 6 of each ultrasound transducer are provided with a firstand a second current supply contact 8, 9. The individual ultrasoundtransducers are electrically connected on the substrate 1 in such a waythat a one-dimensional array of ultrasound transducers is obtained. Thisresults in an array of ultrasound transducers in which each ultrasoundtransducer has a very high piezoelectric coupling coefficient k.

[0079] Embodiment 6

[0080] An array of ultrasound transducers comprises a silicon substrate1 which comprises an insulating layer of Si₃N₄ and SiO₂ on one side. Onthe opposite side, a membrane 2 composed of a layered system ofSiO₂/Si₃N₄ is provided. The substrate 1 has several openings 3 whichadjoin the membrane 2 at one side. A barrier layer 7 of MgO with a (100)orientation is present on the membrane 2. A layer ofPbZr_(0.15)Ti_(0.85)O₃ doped with lanthanum and manganese and with a(100) orientation is grown on the barrier layer 7 so as to form thepiezoelectric layer 5. A first and a second electrode 4, 6 of Pt arepresent on each piezoelectric layer 5. The two electrodes 4, 6 arestructured such that two electrode surfaces on the upper side of thepiezoelectric layer 5 form an ultrasound transducer each time. The firstand second electrodes 4, 6 of an ultrasound transducer are provided witha first and second current supply contact 8, 9, respectively. Theindividual ultrasound transducers are electrically connected on thesubstrate 1 such that a one-dimensional array of ultrasound transducersis obtained. The application of a bias voltage to each individualultrasound transducer at elevated temperatures polarizes the ceramicmaterial in the piezoelectric layer 5 in the field direction. An arrayof ultrasound transducers is obtained thereby in which each ultrasoundtransducer has a very high piezoelectric coupling coefficient k.

[0081] Embodiment 7

[0082] An array of ultrasound transducers comprises a silicon substrate1 which comprises an insulating layer of Si₃N₄ and SiO₂ on one side. Onthe opposite side, a membrane 2 composed of a layered system ofSiO₂/Si₃N₄/SiO₂ is provided. The substrate 1 has several openings 3which adjoin the membrane 2 at one side. A barrier layer 7 of MgO with a(100) orientation lies on the membrane 2. A first electrode 4 of Ti/Ptis provided on the barrier layer 7 wherever there is an opening 3 in thesubstrate 1. The Pt-containing layer of the first electrode 4 is grownso as to have a (100) orientation. A thin nucleating layer ofPbZr_(0.35)Ti_(0.65)O₃ is provided on each first electrode 4. A layer ofPb(Mg_(⅓)Nb_(⅔)) O₃—PbTiO₃ with a (001) orientation is grown on eachnucleating layer so as to form the piezoelectric layer 5. A secondelectrode 6 of Pt is present on each piezoelectric layer 5. The firstand second electrodes 4, 6 of each ultrasound transducer are providedwith a first and second current supply contact 8, 9, respectively. Theindividual ultrasound transducers are electrically to connected on thesubstrate 1 such that a one-dimensional array of ultrasound transducersis obtained. This results in an array of ultrasound transducers in whicheach ultrasound transducer has a very high piezoelectric couplingcoefficient k.

[0083] Embodiment 8

[0084] An array of ultrasound transducers comprises a silicon substrate1 which comprises an insulating layer of Si₃N₄ and SiO₂ on one side. Onthe opposite side, a membrane 2 composed of a layered system ofSiO₂/Si₃N₄/SiO₂ is provided. The substrate 1 has several openings 3which adjoin the membrane 2 at one side. A barrier layer 7 of MgO with a(100) orientation lies on the membrane 2. A first electrode 4 of Ti/Ptis provided on the barrier layer 7 wherever there is an opening 3 in thesubstrate 1. The Pt-containing layer of the first electrode 4 is grownso as to have a (100) orientation. A layer of Pb(Zn_(⅓)Nb_(⅔)) O₃—PbTiO₃with a (001) orientation is grown on each first electrode 4 as thepiezoelectric layer 5. A second electrode 6 of Pt is present on eachpiezoelectric layer 5. The first and second electrodes 4, 6 of eachultrasound transducer are provided with a first and second currentsupply contact 8, 9, respectively. The individual ultrasound transducersare electrically connected on the substrate 1 such that aone-dimensional array of ultrasound transducers is obtained. Thisresults in an array of ultrasound transducers in which each ultrasoundtransducer has a very high piezoelectric coupling coefficient k.

[0085] Embodiment 9

[0086] An array of ultrasound transducers comprises a silicon substrate1 which has an insulating layer of Si₃N₄ and SiO₂ on one side. On theopposite side, a membrane 2 composed of a layered system ofSiO₂/polycrystalline silicon/SiO₂ is provided. The substrate 1 has aplurality of openings 3 which each adjoin the membrane 2 at one side. Abarrier layer 7 of MgO with a (100) orientation lies on the membrane 2.A thin nucleating layer of PbZr_(0.35)Ti_(0.65)O₃ is provided on thebarrier layer 7 wherever there is an opening 3 in the substrate 1. Alayer of Pb(Mg_(⅓)Nb_(⅔))O₃—PbTiO₃ with a (001) orientation is grown oneach nucleating layer so as to form the piezoelectric layer 5. A firstand a second electrode 4, 6 of TiW/Al lie on each piezoelectric layer 5.The two electrodes 4, 6 are structured such that two electrode surfaceson the upper side of the piezoelectric layer 5 form an ultrasoundtransducer each time. The first and second electrodes 4, 6 of eachultrasound transducer are provided with a first and second currentsupply contact 8, 9, respectively. They are electrically connected toother ultrasound transducers on the substrate 1 in such a way that aone-dimensional array of ultrasound transducers is obtained.

[0087] Embodiment 10

[0088] An array of ultrasound transducers comprises a 400 μm thicksubstrate 1 manufactured in a film casting process fromyttrium-stabilized ZrO₂ with an average grain size of 0.4 μm and asuitable organic binder. Openings 3 were made in the substrate 1 in astamping process in a number as required for the envisaged array ofultrasound transducers. The openings 3 penetrated fully through thesubstrate. The substrate 1 was subsequently fully covered with a second,10 μm thick foil of yttrium-stabilized ZrO₂, laminated, and sintered. Asubstrate 1 with a sintered membrane 2 was obtained thereby. A firstelectrode 4 of Pt was provided on the sintered membrane 2 wherever therewas an opening in the substrate 1. The first electrode 4 was coated forapproximately 10% with a nucleating layer of needle-shaped crystals ofBi₄Ti₃O₁₂ with an average diameter of 2 to 5 μm. The needle-shapedcrystals lay flat on the first electrode 4 and thus initiated apreferred direction, acting as crystallization nuclei in the subsequentsintering process. Then Pb(Sc_(⅓)Nb_(⅔))O₃—PbTiO₃ was provided on eachnucleating layer in a thick-film process in the form of a ceramic powderof 30 μm thickness and was sintered in a textured manner at 1275° C. Therespective second electrodes 6 of Ag were subsequently provided. Thefirst and second electrodes 4, 6 were provided with respective first andsecond current supply contacts 8, 9 and connected to other ultrasoundtransducers on the substrate 1 in such a manner that a one-dimensionalarray of ultrasound transducers was obtained.

1. An array of ultrasound transducers which each comprise a substrate(1), a membrane (2), a first electrode (4), a piezoelectric layer (5),and a second electrode (6), said substrate (1) comprising at least oneopening (3) which adjoins the membrane (2) at one side, wherein saidpiezoelectric layer (5) is a textured layer.
 2. An array of ultrasoundtransducers as claimed in claim 1, characterized in that thepiezoelectric layer (5) is a monocrystalline layer.
 3. An array ofultrasound transducers as claimed in claim 1, characterized in that thematerial of one electrode is textured.
 4. An array of ultrasoundtransducers as claimed in claim 1, characterized in that thepiezoelectric layer comprises a material chosen from the groupPb(Zn_(⅓)Nb_(⅔))O₃—PbTiO₃, Pb(Mg_(⅓)Nb_(⅔))O₃—PbTiO₃,Pb(Ni_(⅓)Nb_(⅔))O₃—PbTiO₃, Pb(Sc_(⅓)Nb_(⅔)) O₃—PbTiO₃,Pb(Zn_(⅓)Nb_(⅔))_(1-x-y)(Mn_(½)Nb_(½))_(x)Ti_(y)O₃ (0≦x≦1, 0≦y≦1),Pb(In_(½)Nb_(½))O₃—PbTiO₃, Sr₃TaGa₃Si₂O₁₄, K(Sr_(1-x)Ba_(x))₂Nb₅O₁₅(0≦x≦1), Na(Sr_(1-x)Ba_(x))₂Nb₅O₁₅ (0≦x≦1), BaTiO₃, (K_(1-x)Na_(x))NbO₃(0≦x≦1), (Bi,Na,K,Pb,Ba)TiO₃, (Bi,Na)TiO₃, Bi₇Ti₄NbO₂₁,(K_(1-x)Na_(x))NbO₃—(Bi,Na,K,Pb,Ba)TiO₃ (0≦x≦1),a(Bi_(x)Na_(1-x))TiO_(3−b)(KNbO_(3−c)){fraction (1/2)}(Bi₂O₃—Sc₂O₃)(0≦x≦1, a+b+c=1), (Ba_(a)Sr_(b)Ca_(c))Ti_(x)Zr_(1-x)O₃ (0≦x≦1, a+b+c=1),(Ba_(a)Sr_(b)La_(c))Bi₄Ti₄O₁₅ (a+b+c=1), Bi₄Ti₃O₁₂, LiNbO₃,La₃Ga_(5.5)Nb_(0.5)O₁₄, La₃Ga₅SiO₁₄,La₃Ga_(5.5)Ta_(0.5)O₁₄ andPbZr_(x)Ti_(1-x)O₃ (0≦x≦1) with and without dopants of La, Mn, Fe, Sb,Sr, Ni or combinations of these dopants.
 5. An array of ultrasoundtransducers as claimed in claim 1, characterized in that a barrier layer(7) is provided on that side of the membrane (2) which faces thepiezoelectric layer (5).
 6. An array of ultrasound transducers asclaimed in claim 1, characterized in that the first and the secondelectrodes (4,6) are disposed at opposite ends of the piezoelectriclayer (5).
 7. An array of ultrasound transducers as claimed in claim 6,characterized in that the electrodes (4, 6) are disposed on the samesurface of the piezoelectric layer (5).
 8. An array of ultrasoundtransducers as claimed in claim 6, characterized in that at least onefurther electrode is disposed between the first and second electrodes(4, 6).
 9. An array of ultrasound transducers as claimed in claim 8,characterized in that at least two electrodes are of the same polarityand at least one electrode is of opposite polarity, wherein electrodesof the same polarity are coupled in parallel.
 10. An array of ultrasoundtransducers as claimed in claim 6, characterized in that the electrodes(4, 6) form concentric rings.
 11. An array of ultrasound transducerswhich each comprise a substrate (1), a membrane (2), a first electrode(4), a piezoelectric layer (5), and a second electrode (6), saidsubstrate (1) comprising at least one opening (3) which adjoins themembrane (2) at one side, characterized in that a barrier layer (7) isprovided on that side of the membrane (2) which faces the piezoelectriclayer (5).
 12. An array of ultrasound transducers as claimed in claim11, characterized in that one of the electrodes (4, 6) is a texturedelectrode formed on the barrier layer (7).
 13. An array of ultrasoundtransducers as claimed in claim 11, characterized in that the barrierlayer (7) has a bending stiffness which is less than that of themembrane layer.
 14. An array of ultrasound transducers as claimed inclaim 13, characterized in that the distance between the neutral planeof a transducer and the piezoelectric layer (5) is larger than thethickness of the piezoelectric layer (5).
 15. An array of ultrasoundtransducers as claimed in claim 11, characterized in that the barrierlayer (7) is thicker than the piezoelectric layer (5).
 16. An ultrasoundtransducer which comprises a substrate (1), a membrane (2), a firstelectrode (4), a piezoelectric layer (5), and a second electrode (6),the substrate (1) comprising at least one opening (3) which adjoins themembrane(2) at one side, wherein said piezoelectric layer (5) is atextured layer.
 17. A method of manufacturing an array of ultrasoundtransducers which each comprise a substrate (1), a membrane (2), a firstelectrode (4), a piezoelectric layer (5), and a second electrode (6),the substrate (1) comprising at least one opening (3) which adjoins themembrane(2) at one side, wherein said piezoelectric layer (5) ismanufactured in a textured manner.